Note: Descriptions are shown in the official language in which they were submitted.
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DEVICE FOR MONITORING OF CORROSION
BACKGROUND OF THE INVENTION
Field of the Invention:
The invention under consideration concerns a device for
monitoring corrosion.
Description of the Invention:
The potentials which different metals take on in an
electrolyte in comparison with a reference potential given by a
reference electrode permit important corrosion-chemical statements.
Metals or alloys which can form protective layers in concerned
electrolytes show potentials which can vary in a given potential
range. In these cases, conclusions can be drawn from the poten-
tial as to the electro-chemical condition of the metal. In
particular, it can be deduced whether hole-forming corrosion
is possible or not.
Particularly in the case of heat exchangers, especially
for corrision-endangered condensers of large coolers in steam
power plants with a high capacity, it would be of a high economi-
cal importance if the free corrosion potential could be monitored
during operation. However, this is not possible with the known
arrangements for the reasons discussed hereinbelow.
Normally, water chambers are not accessible during
operation for potential measuring. Also, there are modern water
chambers equipped with protective anodes for the cathodic corro-
sion protection whereby the penetration tubes take on a mixedpotential which is different from the free corrosion potential.
Finally, the free corrosion potential normally developes only
in the inner part in case of long heat exchanger tubes (for
example 10 m with today's types of condensers). Thus, this inner
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part is decisive for the coxrosive behavior of the heat exchanger
tube. However, as can be noted from what has been said above,
this point is practically inaccessible for a measuring probe.
It is now the task of the invention under consideration
to provide a device for the monitoring of corrosion-endangered
tubes, particularly for heat exchangers operating with water,
which overeomes the aforementioned difficulties and which is
also particularly suitable to measure the free corrosion poten-
tial of heat exchangers during operation.
SUMMARY OF TH~ I~VENTION
The present invention pertair~s to an apparatus
for the monitoring of corrosion in at least one metal heat ex-
changer tube through which a corroding medium flows with an
electrical measuring instrument controlled by the condition of
a surface in contact with the corroding medium, which surface
comprises the material of at least one tube to be monitored.
In one aspect the apparatus includes at least one
branch placed para:Llel to the flow in the tube to be monitored
wherein the branch comprises a monitor tube connected via elec-
trical insulating conduit means at its opposite ends to the heatexehanger tube to be monitored. The monitor tube has an inner
cross-section whieh is at least equal to that of the tube to
be monitored sueh that the corroding medium in the monitor tube
flows with the same speed as in the tube to be monitored, the
monitor tube eomprising the same metallic material as the tube
to be monitored. A reference electrode is disposed in electrical
contaet with the corroding medium in the branch and an electrical
eontaet deviee is mounted on the monitor tube with the electrical
measuring instrument being electrically connected with the
reference electrode and with the contac-t device.
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In another aspect the apparatus includes a branch
placed parallel to the flow in the tube to be monitored wherein
the branch comprises at least one monitor tube connected via
eleetrical insulating eonduit means at its opposite ends to the
S heat exehanger tube to be monitored, and the monitor tube having
an inner cross-section which is at least equal to that of the
tube to be monitored such that the eorroding medium in the
monitor tube flows with the same speed as in the tube to be
monitored. The monitor tube comprises the same metallic material
as the tube to be monitored and a pair of electrical contact
devices are mounted on the monitor tube with the electrical
measuring instrument being connected with the monitor tube so
- as to form a closed electrical circuit. In a still further aspect
the apparatus may comprise at least two monitor tubes within a..
insulating line connecting the monitor tubes.
A branch is placed parallel to the flow in the heat
exchanger tube to be monitored. The monitor tubes each comprise
a portion of the branch and the branch has an inner cross-
section which is at least equal to that of the tube to be moni-
tored, the corroding medium in the monitor tubes flowing with thesame speed as in the tube to be monitored. The monitor tubes
comprise the same metallic material as the at least one tube to
be monitored. An electrical eontact deviee is connected to
each of the monitor tubes, the eleetrieal measuring instrument
being eleetrically connected to form one electrical circuit
with the monitor tubes through the contact devices, which
electrical circuit is closed by the corrodins medium.
Among the associated advantages accomplished with the
present invention, there is, first of all, the already mentioned
fact that, the free corrosion potential can be continuously
monitored whereby critical operational conditions for localized
corrosion can be immediately recognized. This represents a
prerequisite in order to take remedial measures in time. Addi-
tionally, the respective system must not be turned off and
opened up for the respective measuring.
Of further significance is the fact that, with a device
according to the invention, the measured potential is the same
as that in the interior even of a very long condenser and heat
exchanger tubes which are not accessible for measuring. This
is due to the fact that the surface of the test tube is exposed
to the same media and to the same operational conditions as the
tubes to be monitored. Furthermore, since the entire device
according to the invention is not smaller at any point than the
inner cross-section of the tubes to be monitored, the advantage
is obtained that the sponge shots of a sponge shot cleaning sys-
tem can also pass through it without any difficulty. Thus, the
condition of the surface of the test tube is also representative
with the temporary operation of a sponge shot cleaning system,
and all operational conditions important for the recognition of
corrosion are covered. Operational methods and conditions which
could lead to corrosion can be recognized and eliminated by the
present invention. Maintenance is also simplified since only
then corrosion-prott~ctive measures (for example, dosage of high
amounts of iron sulphate) are to be taken when it is necessary.
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The present invention thus permits operation of heat exchangers (for example,
condensers) more safely whereby the availability of thermal power plants
is increased.
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~ BRIEF DESCRiPTI~N:OF THE DRAWINGS
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i,~ 5 Various other objects, features and attendant advantages of the
present invention will be more fully appreciated as the same becomes better
`~ understood from the following detailed description when considered in
connection with the accompanying drawings, wherein like reference characters -
designate like or corrsponding parts throughout the several views, and
~10 wherein:
FIGURE~l is a lateral view of two monitoring devices accordi~ng to
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the invention for potential measuring, connected to a schematically illu~trated
: heat exchanger;
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FIGURE 2 shows a top view on the arrangement according to FIGURE l;
: 15 FIGURE 3 is a detailed representation of a special design
according to FIGURES 1 and 2;
FIGURE 4 is a detailed representation of an additional special
design according to FIGURES 1 and 2;
,
`~ FIGURE 5 shows a potential measuring system with a quicksilver/
. calomel reference electrode;
` FIGURE 6 illustrates a copper/copper sulphate reference electrode
: of the present invention;
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FIGURE 7 discloses a monitoring device according to the invention
with a resistance measuring method; and
FIGURE 8 shows a monitoring device according to the invention
with a polarization resistance measuring method.
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DETAILED DESCRIPTION OF T~IE PREFERRED EMBUDIMENTS
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The heat exchanger shown in FIGURES 1 and 2 includes, besides its
heat exchanging portion with the tubes 2 to be monitored, an inlet water
chamber 24 and an outlet water chamber 25. The corroding medium 3, i.e.
the cooling water, enters and leaves the unit according to the arrows. The
inlet water chamber 24 is connected with the outlet water chamber 25 by means
of a large number of heat exchanger tubes 2 (only one of which is shown here
in a dotted line). `~
The water chambers 24 and 25 are connected by one or several
branches running outside the heat exchanger parallel to the heat exchanger
tubes 2. The cooling water flows in these branches with the same speed as
in the heat exchanger tubes 2. In FIGURES 1 and 2, two such branches are
shown because, in the case of the lower branch, the branch-off point at the
water chamber 24 is provided with a screen to retain cleaning shot. In
this manner, the influence of the tube cleaning on the corrosive behavior can
be determined.
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The branches consist of a test tube 1 at both ends of which
insulating plping 8, possibly of soft PVC, is installed which is fastened
to the water chambers 24 and 25 through sockets. The inner diameter of the
. insulating piping 8 and of the test tube 1 is equal to or slightly larger
- 5 than that of the heat exchanger tubes 2. At both ends of the test tube 1,
a stop cock 23 is installed. A T-shaped insulating hollow body 9 is placed
on the test tube 1 for the purpose of holding a reference electrode 4.
' The device shown is based on the fact that a relatively short
piece of tube, i.e. the test tube 1, through which the same cooling water
. 10 flows with the same speed and which consists of the same material as the
heat exchanger tubes 2 to be measured, reaches the free corrosion potential
of the heat exchanger tubes 2 in the interior of the heat exchanger. With
the design of the branch shown in FIGURES 1 and 2, no interfering inlet
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turbulence and no potential-shifting effect of possible protective anodes
can occur.
As is shown in FIGURES 3 and 4, the test tube 1 has, for the purpose
of monitoring the electrical potential it will reach, a small borehole 7
into which reference electrode 4 is placed which, in its turn, is held by a
T-shaped hollow body 9 placed on the test tube 1 in FIGURES 1 to 3. The
test tube 1 is provided with an electrical contact 5 and the free corrosion
potential can be rneasured between test tube i and reference electrode 4 with
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a suitable electrical millivoltmeter 6 with an inner resistance of at
least 106 to 108 ohrrls. If this potential exceeds a certain value (i.e.
" the "localized corrosion potential"), the danger exists that corrosionoccurs in a localized area. The entire branch with the test tube 1 rises
S in the flow direction so that the test tube 1 removes air on its own
during operation and empties itself during a standstill mode. Since the
pressure difference and thevi~c~sity of the corroding medium 3 is a firmly
given factor by the respective heat exchanger, the speed of the medium 3 is
expediently adjusted by varying two dimensions(i.e. length and inner cross-
section)of the test tube 1. However, since the cross-section should be such
that the sponge shots of a cleaning system can pass therethrough, the
necessity of a certain minimum length of the test tube 1 arises. In practice,
the test tube 1 will be made of a piece of the used heat exchanger tubes 2
and its length should be at least approximately e~ual to one half of its
diameter. However, as a rule, this length could amount to 5 - 200 times the
diameter of the tube.
FIGURE 3 shows a test tube 1 through which the corroding medium 3,
for example, cooling water, flows. The test tube 1 is connected with the
heat exchanger by means of the insulating piping 8. Furthermore, a T-
shaped tube is placed on the test tube 1 as an insulatîng hollow body ~ whose
vertical portion 22 is exactly above the corresponding borehole 7 throu'gh
the test tube 1. The insulating hollow body 9 is held on the test tube 1 by
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; means of two O-rings 12 and ld and eontact serew iO. Flnally,
the referenee eleetrode 4 ls eonneeted, in an electrieally
conductlve manner, with che electrical con-tact device 5 througn
- the millivol~meter 6. Instea~ o~ makincJ the contact through
the electrieal contaet device 5, sueh can also be made through
the eontact scr~w L0 to the test tube.
1~ In FIGURE 4, again a test tube 1 is shown through which the
corroding medium 3 flows. In this design, a support tube 17 is slipped over
the test tube 1. In this instance, a lateral recess 21 in the support
tube 17 is above a borehole 7 of the test tube 1. The support tube 17 is
held by means of two 0-rings 12, 14 and a cover 18. A reference electrode
4 is inserted into the lateral recess 21.
The reference electrode 4 contacts the corroding medium 3 throuqh
- the borehole 7 of the test tube 1. In order to be fastened, the reference
L5 electrode 4 is first pressed on an 0-ring 13. Then, the reference electrode
4 can be fastened in a particularly safe and simple manner by means of an
0-ring 13 and a screw tube 20 which, at the same time, makes it impossible
for the reference electrode 4 to slip out during installation by means of a
rotating plate 26 with retaining spring 19. The electrical supply line 27
runs to the reference electrode 4 through the rotating plate 26. The measurin~
instrument 6 (not shown) is connected, on thf~ one hand, to the supply line
27 and, on the other hand to a contact device 5 on the test tube one, with
the contact device 5 ~not shown).
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FIG~IRE 5 represents a potentidl measurin~ system with
a quicksilver/calomel reference electrode which is immersed in
the corroding medium 3 in which there is a metal sample 42. The
reference electrode consists of an electrode unit 31 into which
a narrow small tube 37 is sealed. The electrode unit 31 is also
charged with a saturated KCl solution 29. The narrower small
-tube 37 is filled with quicksilver 30 and solid quicksilver-I-
chloride (Hg2 C12 = calomel) 41 which is soluble with
difficulty. This quicksilver 30 has, in comparison with the
:LO saturated KCl solution, a constant potential which is detected
by the electrical supply line 36.
FIGURE 6 represents a copper/copper sulphate reference
electrode as an additional example of a reference electrode 4.
The electrode uni-t 31 has a charge opening 43 with a membrane 32
on the bottom and is charged with the saturated copper sulphate
solution 33. A copper rod 34 is immersed in this solution
which, in comparison with the copper sulphate solution, has a
constant potential which is detected by the electrical supply
line 36.
Although Figures 5 and 6 have shown a quicksilver/
calomel reference electrode or a copper/copper sulphate
electrode respectively, reference electrodes may also be of
silver/silver chloride or silver/silver sulphate.
The inven-tion under consideration is not only suitable
for the moni-toring of corrosion with the help of measuring the
electrical potential whereby, as has already been explained,
localized corrosion can be detected in time since the invention
is actually also suitable for the rnonitoring of corrosion with
the help of the resistance (i.e. so-called resistance method)
and, additioncLlly, for -the monitoring of corrosion with the help
of the polarization reslstance (i.e. so-called polarization
resistance measuring Method).
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FIGURE 7 shows a special design of the invention in accordance
with the resistance measuring method. The test tube l,which could be
considered as being connected to a heat exchanger (not shown here) by
. means of the insulating pip;ng 8, is arranged to let the corroding
medium 3 flow through it. The test tube 1 also has two contact lugs 5',
5" as contact devices which are attached advantageously on both its ends.
An electric resistance meter (ohmeter) 6 is provided as an electrical
measuring instrument whose connections are attached to the contact lug 5",
5" through supply lines 27. The resistance measuring method is based on the
recognition of the corrosion-caused reduction in the cross-sections of the
tube 2 to be monitored. Tf, for example, metal removal, caused by corrosion,
takes place, the electrical resistance of the test tube 1 increases and
thus represents a measurable dimension for the progress of the corrosion.
In FIGURE 8, a special design of the invention for the monitoring
of corrosion with the help of measuring the polarization is disclosed.
As is shown in FIGURE 8, this design requires at least two individual test
tubes 1', 1" which are connected with each other by an insu1ation line 8',
possibly a piece of a PVC hose. The corroding medium 3 flows through the
two test tubes 1', 1". The tube composed of two test tubes 1', 1" is to
2û be considered as being connected to a heat exchanger, (not shown here) bymeans of additional insulating piping 8. The individual test tubes 1', 1"
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have a contact lug 5', 5", respectively, as a contact device which i5 con-
nected with the polarization resistance measuring instrument 6 as the elec-
trical rneasuring instrument through the electrical supply line 27. Such a
measuring instrument 6 is, for example, available from the company of
Armin Lùdi, Bellevuestr. 112, 3028 Spiegel-Bern, under the trade m~rk Winking.
The polarization resistance measu~ing is based on detecting the transfer
resistance between metal and electrolyte whichdetermines the corrosion speed.
The current which is expediently produced by a current source
installed in the resistance meter (ohmeter) leaves, for example, the left
test tube and enters again into the right test tube through the c~ m~d~n9
medium. This discharge and admission work to be handled by the elé/c~rons
with the discharge metal/corroding medium and with the admission corroding
me~ium/metal is dependent on the surface property of the metal surface, such
as for example, a corrosion-caused covering layer or corrosion-caused decom-
position. The more difficult the work is which the electrons must handle,
the higher then also is the electric resistance of these electric circuits
which can be practically measured.
It goes without saying that, in a single branch9 if so desi~ed, all
three types of measuring devices shown in this application or a combination
of these three types can be applied by simply placing the il~dividual devices
one behind the other in a single branch and connecting them in series.
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Finally, the particularly simple design of the invention should
be pointed out which results in a very low price despite the great and
surprising effectiveness of the invention. The special design with
potential measuring permits demonstration of (also in case of nletals being
covered with a relatively dense covering layer) localized corrosion occuring
under the covering layer. Of particular advantage is, in the case of all
three designs, the possibility of continuous monitoring also during operation
of the heat exchanger~. The detection is thus effected at a time when the
damage is still easily avoidable. Besides the possibility of a connection
in parallel, several measurements can also be performed by connecting several
test tubes in series in a single branch. The test tubes can be easily
installed and removed,even during operation of the heat exchanger.
Obviously, many modifications and variations of the present inventio~
are possible in light oF the above teachings. It is therefore to be understoo
that within the scope of the appended claims, the invention may be practiced
rtherw e than as specificd11y described herein. :m: i~5
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